As disclosed in Patent literature 1, there is known WAVE (Wireless Access in Vehicular Environments) as a telecommunication standard used for a communication device (in-vehicle device, hereinafter) used in a vehicle to perform direct communication with a different communication device. A communication partner for a subject in-vehicle device includes a different in-vehicle device or a communication device (roadside device, hereinafter) present in a road. The roadside device performs direct communications with the subject in-vehicle device, providing a predetermined service to an occupant of a host vehicle using the subject in-vehicle device.
WAVE specifies two kinds of communication channels of a control channel (CCH, hereinafter) and a service channel (SCH, hereinafter); two kinds of communication channels are used for respective communication devices to perform direct communication. An SCH is a communication channel used for transmission and reception of the information for providing and enjoying services; a CCH is a communication channel used for transmission and reception of the information for starting the communication using an SCH.
Further, WAVE designates alternately a CCH time zone for communication using the CCH and an SCH time zone for communication using the SCH, with predetermined time intervals (each 50 milliseconds, for instance).
Each communication device designates basically the CCH as a communication channel (communication-target channel) being a transmission/reception target to perform the communication using the CCH while the current clock time is within the CCH time zone. In performing the communication using an SCH, the communication-target channel is switched from the CCH to an intended SCH at the point of time when the CCH time zone transitions into the SCH time zone. At the point of time when the SCH time zone is completed, the communication-target channel is returned to the CCH.
Whether the current clock time is within the CCH time zone or the SCH time zone is determined for each communication device based on the clock time information held in each communication device. There may be arising an error in clock time information between a transmitting device that is a communication device going to transmit data and a receiving device that is a communication device going to receive the data. Such a case involves an error in the point of time for switching the communication-target channel, possibly posing a communication failure. This communication failure signifies an event that causes a receiving device to fail to receive the data transmitted by a transmitting device.
In order to suppress such an error in the clock time information, each in-vehicle device performs a synchronization process to synchronize the clock time information held by itself with a predetermined reference clock time, using a PPS signal (PPS: Pulse Per Second) outputted each second from a GNSS receiver or a TA frame (TA: Timing Advertisement) distributed from a roadside device to advertise the point of time when the seconds change from a previous second to a new second.
This GNSS receiver is a device receiving the radio waves from GNSS satellites used in a well-known global navigation system (GNSS, hereinafter). The GNSS receiver receiving the radio waves from GNSS satellites outputs a PPS signal each second.
In addition, WAVE specifies a guard interval that is a time zone for prohibiting the transmission of data, before and after the point of time when the communication time zone such as the SCH time zone and the CCH time zone switches. This is because just before or just after the point of time when the communication time zone switches, the respective communication-target channels in a transmitting device and a receiving device may fail to accord with each other, due to the errors in the clock time information held by each communication device.